[hpsdr] LTC 2208 A/D Thermal Characteristics

Wed Jan 14 08:27:41 PST 2009

Hi

I don't know if this can add some data to the 
discussion. I measured temperatures in my mercury 
with a (old but unused)  Tektronix temperature 
probe with an old Tektronix 2465DMS scope and 
with an ambient temperature of 18.5 C it gives 
44.7 C (after an hour running) . The probe is in 
contact with the center of the IC. The ambient 
temperature measured with the probe is quite 
close to another mercury (btw) thermometer I have (which is also uncalibrated)

Eduardo/EA2BAJ

At 12:20 14/01/2009, Rob Frohne wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>
>Hi Scott,
>
>That extremely small thermal resistance cannot 
>be between the junction and the place where 
>Charles and others are measuring the temperature.
>It must be between the junction and the part of 
>the case under the chip where it is supposed to 
>be soldered to a good heat sink.  I am not 
>saying there is a problem, only that your 
>analysis is not conclusive, given the 
>measurements that we can fairly easily 
>perform.  Here are the words of wisdom from the 
>LTC2208 data sheet regarding heat considerations:
>
>Most of the heat generated by the LTC2208 is transferred
>from the die through the bottom-side exposed pad. For
>good electrical and thermal performance, the exposed
>pad must be soldered to a large grounded pad on the PC
>board. It is critical that the exposed pad and all ground
>pins are connected to a ground plane of sufficient area
>with as many vias as possible.
>
>The interesting thing is that their example 
>board didn't contain any vias to the ground 
>plane that I could see anyway, so it seems that 
>they didn't follow their own advice, or maybe I'm blind.  :-)
>
>In any case, increasing the heat sinking will 
>not cause the device to operate worse, only 
>perhaps a little better, so I hesitate to 
>discourage folk like Francis from doing as they 
>have with more heat sinking at least at this point.
>
>73,
>
>Rob, KL7NA
>
>Scott Cowling wrote:
>>***** High Performance Software Defined Radio Discussion List *****
>>
>>Hi Mercurians,
>>
>>The thermal resistance of the LTC 64-pin UP 
>>package (the one that the LTC2208CUP comes in) 
>>is specified at 0.24°C/W from the junction to the case.
>>
>>The maximum allowed operating junction temperature is 125°C.
>>
>>The A/D dissipates roughly 1.25W at 3.3V and 
>>1.45W at 3.6V. We are running at 3.3V, but lets 
>>assume a worst-case power dissipation of 1.5W, 
>>which is higher that it will ever be.
>>
>>1.5W * 0.24°C/W = 0.36°C temperature rise from the case to the junction.
>>
>>That means the junction temperature, T(j), will 
>>be only about 1/3 of a degree higher than 
>>whatever temperature you measure on the case.
>>
>>Since the maximum junction temperature is 
>>125°C, the case can get as hot as 124°C WITHOUT 
>>EXCEEDING THE MAXIMUM ALLOWED JUNCTION TEMPERATURE.
>>
>>Since the case temperatures mentioned here are 
>>in the 50°C to 70°C range, the chip die is 
>>operating at well below the maximum allowable temperature.
>>
>>While I will never argue against "cooler is 
>>better", I will argue against making a problem where there is none.
>>
>>Could we make it cooler? Sure we could. Do you 
>>want to add noise by hooking up a BIG ground 
>>plane to the pad on the underside of the chip? 
>>Maybe. Designs are about tradeoffs. The 
>>designers traded off board area, noise pickup 
>>and thermal resistance and came up with a design that works.
>>
>>It is cool *enough*
>>It is quiet *enough*
>>It fits on the board
>>It works
>>
>>That doesn't mean that it is necessarily the 
>>optimum choice. By all means, modify it, change 
>>it, make it better; that is what HPSDR is all 
>>about. There are many solutions, some of which 
>>are certainly better that the one that we 
>>chose. But please don't call it "broken" just 
>>because it feels hot to your finger. :-)
>>
>>Here are some reference points:
>>At 50°C, it takes 2 minutes to give you a third degree burn.
>>At 60°C, it takes 5 seconds to inflict a third-degree burn.
>>At 70°C, it takes only 1 second to give you a third degree burn.
>>source: http://www.cqc.state.ny.us/newsletter/estime.htm
>>
>>It's hot! Be careful!
>>
>>73,
>>Scotty WA2DFI
>>
>>
>>
>>
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>--
>Rob Frohne, Ph.D., P.E.
>E.F. Cross School of Engineering
>Walla Walla University
>100 SW 4th Street
>College Place, WA 99324
>(509) 527-2075                  http://people.walllawalla.edu/~rob.frohne
>
>
>
>
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